The present invention relates to a light source apparatus for use in an optical apparatus such as an image projection apparatus, an exposure apparatus, and a copier, and more particularly, to a cooling structure in a light source apparatus.
An arc tube as in an ultrahigh pressure mercury lamp is used in a light source for use in an optical apparatus as described above. When such an arc tube is used, a cover member (an explosion-proof member) made of a light-transmissive material such as glass is disposed in front thereof in order to prevent any breakage of the arc tube from scattering its fragments or mercury outside. Thus, the inner side of a reflector for reflecting light from a light emitter in a predetermined direction to collect the reflected light is an almost sealed structure such that the heat of the arc tube serving as a heat source is not exhausted easily.
On the other hand, to increase the operating pressure of the arc tube to allow the arc tube to offer desired light emitting performance, the temperature of the arc tube needs to be increased. However, too high a temperature of the arc tube cannot guarantee operation thereof over a long time. In other words, unless the temperature of the arc tube is maintained in an appropriate range, a devitrification phenomenon of the arc tube occurs such as blackening and whitening or the operation is destabilized. It is thus necessary to perform proper cooling for the arc tube (see Japanese Patent Laid-Open No. 8 (1996)-314011 and Japanese Patent Laid-Open No 2002-245842).
In a cooling structure disclosed in Japanese Patent Laid-Open No. 8 (1996)-314011, an exhaust fan is provided at the back of a lamp unit, and air is introduced from an upper front portion of the lamp unit and exhausted outside a housing from a lower portion of the lamp unit through an exhaust port provided for the housing.
In a cooling structure disclosed in Japanese Patent Laid-Open No 2002-245842, air in a lamp container is exhausted by a blower provided at the back of the lamp to flow air for cooling in the lamp container. Part of the air for cooling in the lamp container flows into a reflector of the lamp unit from an air inlet formed in the front of the lamp body, passes through the reflector, and is exhausted from an air outlet formed in a neck portion of the reflector.
In the cooling structure proposed in Japanese Patent Laid-Open No. 8 (1996)-314011, however, cooling wind smoothly flows near an opening of the reflector, but does not easily reach an arc tube provided near a neck portion of the reflector. This causes the problem of poor efficiency in cooling the arc tube.
In the cooling structure disclosed in Japanese Patent Laid-Open No. 2002-245842, the exhaust hole provided between the neck portion of the reflector and the arc tube is close to the portion where the arc tube is attached to the reflector, so that it is not possible to form a sufficient opening for the exhaust hole which is provided for exhausting high-temperature air stored within the reflector. Thus, the structure has the problem of poor efficiency in cooling the arc tube.
While the cooling efficiency can be improved by increasing the revolutions of the exhaust fan or the size of the exhaust fan, noise or the size of the entire apparatus may be increased in such a case.
The ultrahigh pressure mercury lamp is formed of an arc tube which has a cathode and a anode disposed opposite to each other in a glass tube which is filled with gas and mercury, a reflector having a reflecting mirror for changing light from the arc tube into collimated light, and an attaching member for attaching the arc tube to the reflector.
As shown in FIG. 16, a generally cylindrical arc tube 901 is typically fixed to a reflector 902 via an attaching member 903. Specifically, the arc tube 901 is placed at the center of the reflector 902 and fixed to the attaching member 903 such that the focal point of the surface forming the reflecting mirror within the reflector 902 matches a light-emitting portion of the arc tube 901. This can provide collimated light with high efficiency in collecting light.
The arc tube 901 has a spherical portion (the light-emitting portion) 901a, a cathode seal portion 901b, an anode seal portion 901c, and a brace welding portion 901d. Cooling conditions (recommended temperature ranges) are set for the respective portions such as a temperature range from 900 to 1000 C.° above the spherical portion 901a, a range of 900±20 C.° below the spherical portion 901a, and a range of 420 C.° or lower in the cathode and anode seal portions 901b and 901c, by way of example.
For this reason, a conventionally disclosed structure is formed to supply wind near the opening of the reflector to cool the arc tube as in Japanese Patent Laid-Open No. 2000-82322. Specifically, as shown in FIG. 16, wind is supplied from an intake port 904 near the opening of the reflector and flowed along the reflecting surface of the reflector 902, and the air which cooled the spherical portion 901a is exhausted from an exhaust port 905 closer to the attaching member 903.
Japanese Patent Laid-Open No 2000-21230 has disclosed a structure in which air is supplied from a portion closer to an attaching member 903 at the back of a reflector 902 to cool an arc tube 901 as shown in FIG. 17.
To use a discharge lamp for a long time and provide stable emission of light, it is essential not only to realize a cooling structure which cools the high pressure mercury discharge lamp such that each portion of the high pressure mercury discharge lamp is operated in the recommended temperature range but also to ensure that mercury is adhered to a cathode when the lamp is lit.
In view of those points, while the structure disclosed in Japanese Patent Laid-Open No 2000-82322 sufficiently cools the anode seal portion 901c closer to the leading end of the arc tube 901, the spherical portion 901a is at the highest temperature in the arc tube 901 and the wind after it cools the spherical portion 901a is at high temperature with reduced cooling ability. It is difficult for such wind to sufficiently cool the cathode seal portion 901b closer to the attaching member. For example, when the cathode is disposed closer to the attaching member, the cathode seal portion 901b may be at high temperature to melt and break the metal portion.
In addition, since a reduction in temperature in the cathode seal portion is slower than that in the anode seal portion and the spherical portion, liquid mercury changed from the solid state does not adhere to the cathode but adheres to the anode or a glass wall surface of the spherical portion, the temperature of which is reduced below the boiling point.
Consequently, the cathode is exposed when the lamp is lit next, and the abovementioned phenomenon is repeated to sputter the cathode, which easily causes the phenomenon of blacking. In other words, the life of the lamp is reduced.
In the structure disclosed in Japanese Paten Laid-Open No. 2000-21230, when the anode is disposed at the back of the reflector, it is difficult to cool the cathode since it is downwind of the cooling wind. Mercury is unlikely to adhere to the cathode in cool down after the lamp is turned off, resulting in the blacking phenomenon. In contrast, when the cathode is disposed at the back of the reflector, the temperature of the cathode is first reduced after the lamp is turned off since it is upwind. The condensed mercury can adhere to the cathode to achieve an ideal state.
In the structure, however, the air is supplied from the back of the reflector with a narrow opening, so that a significant amount of pressure is lost when the air passes through an intake port 905. This leads to the problem of failing to supply cooling wind enough to cool the spherical portion 901a of the arc tube 901 if a high-power lamp is used.